FIG. 10 is a cross-sectional view illustrating a prior art optical integrated circuit comprising a laser diode and a photodiode disclosed in, transactions of The Japan Society of Applied Physics, autumn, 1986, P. 178, 29a-T-5. In the figure, reference numeral 1 designates a first conductivity type, for example, n type, InP semiconductor substrate having a thickness of 100 .mu.m and a carrier concentration of 5.0.times.10.sup.18 cm.sup.-3. Reference numerals 1a and 1b designate n type InP semiconductor layers serving as lower cladding layers having a thickness of 1 .mu.m, and a carrier concentration of 1.0.times.10.sup.18 cm.sup.-3 disposed on the semiconductor substrate 1. Reference numerals 2a and 2b designate undoped In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y semiconductor layers serving as active layers having a thickness of 0.1 .mu.m, and an energy band gap narrower than that of the n type semiconductor lower cladding layers 1a and 1b. The undoped semiconductor layers 2a and 2b are disposed on the n type InP semiconductor lower cladding layers 1a and 1b. Reference numerals 3a and 3b designate p type InP semiconductor layers serving as upper cladding layers, having a thickness of 1.0 .mu.m and a carrier concentration of 1.0.times.10.sup.18 cm.sup.-3. The energy band gaps of the layers 3a and 3b are larger than those of the semiconductor layers 2a and 2b and these layers are disposed on the semiconductor layers 2a and 2b, respectively. Reference numeral 4 designates an etched groove penetrating through the semiconductor layers 1, 2, 3 and reaching the semiconductor substrate 1. The etched groove 4 has a depth of 3 .mu.m and the minimum width of 5 .mu.m at the lowermost part of the groove and the maximum width of 10 .mu.m at the uppermost part of the groove. Reference numeral 5 designates a laser diode comprising the semiconductor layers 1a, 2a, 3a and the substrate 1. Reference numeral 6 designates a photodiode comprising the semiconductor layers 1b, 2b, 3b and the semiconductor substrate 1. Reference numeral 12c designates a common n side electrode disposed on the n type semiconductor substrate 1. Reference numeral 2a designates a p side electrode for the laser diode disposed on the p type upper cladding layer 3a. Reference numeral 12b designates a p side electrode for the photodiode disposed on the p type upper cladding layer 3b. One of the side surfaces of the groove 4 at the side of the laser diode 5 is perpendicular to the substrate to form a resonator facet and the other surface at the side of the photodiode 6 is oblique to reflect part of the laser light L emitted from the laser diode which does not enter the photodiode 6 in an upward direction. Therefore, the laser light L, which is emitted from the laser diode 5 and then reflected by the obliquely inclined surface of the photodiode 6, does not again enter into the photodiode 6.
Next, an operation will be described.
When a forward voltage is applied between the electrode 12c at the side of the n type semiconductor substrate 1 and the electrode 12a at the side of p type semiconductor layer 3a for the laser diode 5, electrons and holes as carriers are injected into the semiconductor layer 2a serving as an active layer having a narrower energy band gap than those of the lower and upper cladding layers 1a and 3a, and light having a wavelength corresponding to that energy band gap is Generated in the active layer 2a. This light is amplified by being repeatedly reflected at the resonator facets 5a and 5b at the right and left sides of the laser diode 5, and a laser oscillation occurs to generate laser light. Most of the laser light is emitted from the resonator facet 5b shown at the left side of FIG. 10 and the laser light L other than that is emitted as monitor light from the resonator facet 5a shown at the right side of the figure. The light L emitted from the resonator facet 5a as the monitor light is absorbed in the semiconductor layer 2b in the photodiode 6, having the same narrow energy band gap as the semiconductor layer 2a and narrower than those of the lower and upper cladding layers 1b and 3b, and electrons and holes are generated in the semiconductor layer 2b of the photodiode 6. At this time, when a reverse direction voltage is applied between the n type semiconductor substrate 1 and the p type semiconductor layer 3b, a current on proportion to the intensity of the light which is incident on the photodiode 6 is detected, and the light intensity of the laser diode 5 is thus monitored.
Next, a method for producing the device of FIG. 10 will be described. First of all, the InP, InGaAsP, and InP semiconductor layers 1, 2, and 3 are successively formed on the n type InP semiconductor substrate 1 by, for example, MOCVD (Metal Organic Chemical Vapor Deposition). Then, the etched groove 4 is formed by, for example, RIE (Reactive Ion Etching) employing chlorine (Cl.sub.2) gas. Here, the etched surface 5a at the side of the laser diode 5 is required to be smooth and perpendicular to the semiconductor substrate 1. This is because this etched surface 5a constitutes a resonator of the laser diode 5 with the facet 5b of the laser diode 5 shown at left side of FIG. 10, and this resonator of the laser diode 5 is required to ensure a stable laser oscillation.
In this prior art optical integrated circuit having a structure as described above, in order to attain an improved characteristic of the laser diode, the etched surface is required to be perpendicular to the semiconductor substrate 1 and also smooth. However, there are problems as in the following. Although when a prior art laser diode is produced separately, good quality resonator facets can be produced by cleaving, when the prior art optical integrated circuit having a photodiode together with a laser diode as shown in FIG. 10 is manufactured, etching has to be carried out to produce the groove between the laser diode and the photodiode which also constitutes a resonator facet. This has resulted in great difficulty in obtaining a good quality etched surface by a conventional etching technique. In other words, when etching is carried the crystal at the etched surface is damaged and the reliability of the laser diode is deteriorated. Thus, the improved characteristic of the laser diode cannot be expected in the optical integrated circuit of the above-described structure.